Step by step
The question
Question: find tan A, when sin A = 10/26.
1 Draw the triangle
sin, cos and tan belong to an angle of a right triangle.
So we draw a right triangle with the angle A.
2 Use what sin A means
The opposite side is across from A.
We write opp, adj and hyp for short.
The adjacent side is next to A.
It is not the hypotenuse.
The hypotenuse is the longest side.
It is across from the right angle.
sin A is the opposite side over the hypotenuse.
The question says sin A = 10/26.
So opp over hyp is 10/26.
Every right triangle with the angle A has the same shape.
A bigger one could have 20 and 52.
20/52 is 10/26 too.
So we can use the small one: opp = 10 and hyp = 26.
3 Find the missing side
The adjacent side is missing.
We call it x.
Pythagoras: in a right triangle, the two short sides squared add up to the hypotenuse squared.
Now put the numbers in: opp = 10, adj = x and hyp = 26.
10² means 10 times 10.
That is 100.
26² means 26 times 26.
That is 676.
Write the squares: 100 + x² = 676.
We want x² by itself.
The 100 is in the way.
To remove 100, we take away 100.
We do it on both sides.
Why both sides?
The = sign means both sides are the same amount.
If we change only one side, they are not the same any more.
100 and − 100 make zero.
Cross them out.
676 minus 100 is 576.
x² is 576.
So x times x is 576.
That means x is the square root of 576.
The square root of 576 is the plus number that times itself makes 576.
24 times 24 is 576.
−24 times −24 is 576 too, but the √ sign always means the plus one.
So x = 24.
4 Write tan A
tan A is the opposite side over the adjacent side.
Put in opp = 10 and adj = 24.
10 and 24 can both be divided by 2.
We divide the top and the bottom by 2: the fraction stays the same.
10 divided by 2 is 5.
24 divided by 2 is 12.
So tan A = 5/12.
Careful!
The hypotenuse is never in tan.
tan uses the two short sides: opp and adj.
5 Check
Let's check with the calculator.
Set the calculator to degrees first.
The sin⁻¹ button turns sin A back into the angle: A is about 22.6°.
tan 22.6° is about 0.42.
And 5/12 is about 0.42.
The same.
So tan A = 5/12 is right!
Let's look back.
First: draw the triangle.
Then: use what sin A means.
Then: find the missing side.
Then: write tan A.
Last: check.